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PORTLAND, Ore. The mysterious increase in the acceleration of the universe, when intuition says it should be slowing down, is postulated to be caused by dark energy"dark" because it is undetected. Now a group of scientists in the international collaboration Essence has suggested that a quantum mechanical interpretation of Einstein's proposed "cosmological constant" is the simplest explanation for dark energy. The group measured dark energy to within 10 percent.
Engineers have already proposed harnessing quantum mechanical forces related to dark energy, such as the force postulated by the Dutch scientist Hendrick Casimir in 1948. Hendrick reasoned that photons spontaneously appearing in space a quantum mechanical explanation for dark energy could drive tiny microelectromechanical system plates together. The Casimir force has since been measured by both Ephraim Fischbach, a professor at Purdue University, and Steve Lamoreaux at Los Alamos National Laboratories.
The Essence team members measured the ultimate source of these tiny forcesthe dark energy itselfwith data from 60 of the oldest galaxies in the universe. The group used the Cerro-Tololo Interamerican Observatory's 4-meter telescope to collect the data over four years. They obtained confirmation of their measurements from the 8.2-meter VLT (Very Large Telescope) run by the European Southern Observatory and the 6-meter Magellan telescope, both in Chile; as well as the 8-meter Keck telescope and the 10-meter Gemini telescope, both in Hawaii.
The 38 Essence researchers, who cooperated in countries spanning four continents, concluded that dark energy is the pressure exerted by empty space. From a quantum mechanical perspective, empty space is unstable. According to statistics, photons and subatomic particles pop into the vacuum of space in a way that shows that "empty" is only an approximation: Space actually comprises a statistical soup of particles and antiparticles that are in a constant state of creation. Today scientists can demonstrate this by pumping the gases out of any empty chamber. After every atom has been pumped out, particles begin to percolate into existence in a process called vacuum fluctuation.
"The energy of the vacuum and vacuum fluctuations are related, but I would hesitate to say they are precisely the same thingwe are not theorists, but just analyzed the data," said Tamara Davis, a lead researcher on the University of Copenhagen team. "What we did here was analyze the latest available data to test all the more exotic models of the universe that do not use vacuum energy as the source of the universe's acceleration, and showed that at the moment there is no reason to prefer these models over the simplest model in which the acceleration is caused by a quantum mechanical interpretation of Einstein's cosmological constant." Davis worked with fellow researcher Jesper Sollerman at the University of Copenhagen's Danish Dark Cosmology Centre.
Einstein proposed his cosmological constant to support a static, rather than expanding, hypothesis of the universe. But with the quantum mechanical explanation of vacuum energy as the most likely cause of the expansion of the universe, the researchers propose that, in an otherwise normal theory of gravity, the cosmological constant is a measurement of the background of gravitational pressure.
"What we measured was the equation-of-state of the dark energy, which is just an expression telling you how the pressure of the dark energy is related to its density," said Davis. "We showed that at the moment there is no reason to prefer any but the simplest model in which the acceleration is caused by a cosmological constant."
Davis and colleagues will continue to accumulate observations, especially of very old universes where the differences between then and now are most pronounced. Also Davis plans to study the new results from the world's biggest superatom smasherthe Large Hadron Collider .
"The Large Hadron Collider in Cern, which opens next year, will hopefully give us a lot of new and interesting information about fundamental physics that can inform our models of the universe," said Davis. "This will be an example of how knowledge of the very tinyquantum physicscan teach us about the very largethe universe."
According to the calculations of Davis and other astrophysicists worldwide, unless the vacuum itself exerts the negative pressure observed, then the universe must otherwise be composed of as much as 70 percent dark energy. The observations made by the Essence researchers at the Danish Dark Cosmology Centre and elsewhere have determined that by observing light from dying stars, which was emitted when the universe was about half its current age, the best model to explain the increasing acceleration of the universe is Einstein's 1917 proposal of a cosmological constant, as interpreted by modern quantum mechanical theory.